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+;;; test-custom-whitespace-collapse.el --- Tests for cj/--collapse-whitespace -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--collapse-whitespace function from custom-whitespace.el
+;;
+;; This function collapses whitespace in text by:
+;; - Converting all tabs to spaces
+;; - Removing leading and trailing whitespace
+;; - Collapsing multiple consecutive spaces to single space
+;; - Preserving newlines and text structure
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--collapse-whitespace)
+;; to avoid mocking region selection. This follows our testing best practice
+;; of separating business logic from UI interaction.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+;; Now load the actual production module
+(require 'custom-whitespace)
+
+;;; Test Helpers
+
+(defun test-collapse-whitespace (input-text)
+  "Test cj/--collapse-whitespace on INPUT-TEXT.
+Returns the buffer string after operation."
+  (with-temp-buffer
+    (insert input-text)
+    (cj/--collapse-whitespace (point-min) (point-max))
+    (buffer-string)))
+
+;;; Normal Cases
+
+(ert-deftest test-collapse-whitespace-multiple-spaces ()
+  "Should collapse multiple spaces to single space."
+  (let ((result (test-collapse-whitespace "hello    world")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-multiple-tabs ()
+  "Should convert tabs to spaces and collapse."
+  (let ((result (test-collapse-whitespace "hello\t\t\tworld")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-mixed-tabs-spaces ()
+  "Should handle mixed tabs and spaces."
+  (let ((result (test-collapse-whitespace "hello \t  \t world")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-leading-trailing ()
+  "Should remove leading and trailing whitespace."
+  (let ((result (test-collapse-whitespace "   hello world   ")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-tabs-leading-trailing ()
+  "Should remove leading and trailing tabs."
+  (let ((result (test-collapse-whitespace "\t\thello world\t\t")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-multiple-words ()
+  "Should collapse spaces between multiple words."
+  (let ((result (test-collapse-whitespace "one   two    three     four")))
+    (should (string= result "one two three four"))))
+
+(ert-deftest test-collapse-whitespace-preserve-newlines ()
+  "Should preserve newlines while collapsing spaces."
+  (let ((result (test-collapse-whitespace "hello   world\nfoo   bar")))
+    (should (string= result "hello world\nfoo bar"))))
+
+(ert-deftest test-collapse-whitespace-multiple-lines ()
+  "Should handle multiple lines with various whitespace."
+  (let ((result (test-collapse-whitespace "  hello   world  \n\t\tfoo    bar\t\t")))
+    (should (string= result "hello world\nfoo bar"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-collapse-whitespace-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-collapse-whitespace "")))
+    (should (string= result ""))))
+
+(ert-deftest test-collapse-whitespace-single-char ()
+  "Should handle single character with surrounding spaces."
+  (let ((result (test-collapse-whitespace "   x   ")))
+    (should (string= result "x"))))
+
+(ert-deftest test-collapse-whitespace-only-whitespace ()
+  "Should handle text with only whitespace (becomes empty)."
+  (let ((result (test-collapse-whitespace "   \t   \t   ")))
+    (should (string= result ""))))
+
+(ert-deftest test-collapse-whitespace-no-extra-whitespace ()
+  "Should handle text with no extra whitespace (no-op)."
+  (let ((result (test-collapse-whitespace "hello world")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-collapse-whitespace-single-space ()
+  "Should handle text with already-collapsed spaces (no-op)."
+  (let ((result (test-collapse-whitespace "one two three")))
+    (should (string= result "one two three"))))
+
+(ert-deftest test-collapse-whitespace-very-long-line ()
+  "Should handle very long lines with many spaces."
+  (let ((result (test-collapse-whitespace "word      word      word      word      word")))
+    (should (string= result "word word word word word"))))
+
+(ert-deftest test-collapse-whitespace-multiple-newlines ()
+  "Should preserve multiple newlines while removing spaces."
+  (let ((result (test-collapse-whitespace "hello   world\n\n\nfoo   bar")))
+    (should (string= result "hello world\n\n\nfoo bar"))))
+
+(ert-deftest test-collapse-whitespace-spaces-around-newlines ()
+  "Should remove spaces before/after newlines."
+  (let ((result (test-collapse-whitespace "hello   \n   world")))
+    (should (string= result "hello\nworld"))))
+
+(ert-deftest test-collapse-whitespace-empty-lines ()
+  "Should handle empty lines (lines become empty after whitespace removal)."
+  (let ((result (test-collapse-whitespace "line1\n   \nline2")))
+    (should (string= result "line1\n\nline2"))))
+
+;;; Error Cases
+
+(ert-deftest test-collapse-whitespace-start-greater-than-end ()
+  "Should error when start > end."
+  (should-error
+   (with-temp-buffer
+     (insert "hello world")
+     (cj/--collapse-whitespace (point-max) (point-min)))
+   :type 'error))
+
+(ert-deftest test-collapse-whitespace-empty-region ()
+  "Should handle empty region (start == end) without error."
+  (with-temp-buffer
+    (insert "hello world")
+    (let ((pos (/ (+ (point-min) (point-max)) 2)))
+      (cj/--collapse-whitespace pos pos)
+      ;; Should complete without error and not change buffer
+      (should (string= (buffer-string) "hello world")))))
+
+(provide 'test-custom-whitespace-collapse)
+;;; test-custom-whitespace-collapse.el ends here